Topside structures of offshore installations have to support heavy process plant dealing with large volumes of oil and gas under high pressure. Many of these platforms have to be operated in very remote areas in a harsh environment with little supporting infrastructure. It is therefore necessary to design these high risk installations to various types of extreme loadings. One of these extreme scenarios is blast loading from a possible hydrocarbon explosion. Although this is a comparatively low frequency accidental event, it has the capability to cause major fatalities to personnel and serious structural damage which could lead to the complete loss of the platform. At present, most topside structures are designed based on working stress design (WSD), or load and resistance factor design (LFRD), which is quite safe but not economical due to the uncertain extent of the levels of protection. For these reasons, a Performance Based Design Methodology is proposed, which emphasizes the structure’s predictable behaviour and the protection of personnel and assets. The end result will be an optimum design which satisfies the function of a system without compromising safety. A performance based design guideline for the assessment of topside structures subjected to blast loading is proposed. The guideline, which reasonably incorporated some statistical findings, simplifies the evaluation of performance levels for the topside structure without quantitative risk assessment (QRA) data. The assessment of topside structures is not complete if the proper behaviour and response is not fully understood. It has been shown in the study that the roles of secondary structural members i.e. deck plates and stringer beams cannot be overlooked. Having substantial deformations on secondary members averts severe damage on primary members. A simplified deck plate analytical model is proposed and the optimum slenderness ratio for deck plate design is recommended. Although accuracy of the proposed analytical method is found slightly offset from the finite element result at extreme overpressure, the model is straightforward and provides a quick method to assess the deck plate capacity. The study has also highlighted the weakness of sniped bottom flanges for stringer beams, a necessary condition to facilitate practical fabrication. This shortcoming is overcome by strengthening with angles, a novel idea which is simple and practical with minimum interference to the existing structural configuration. Based upon a typical topside framing, the performance level of the topside is evaluated for reference which can be applied to other topsides. The study has investigated a number of mitigation techniques for improving beam to beam connections. The techniques comprise studies based on some conventional approaches, typical fabrication methods and a new proposal with tubular braces. Finally, the effect of equipment on the topside structure is investigated and recommendations are made to minimise unnecessary damage.